Osmotically driven membrane processes (ODMPs) such as forward osmosis (FO) and pressure-retarded osmosis (PRO) have numerous applications including seawater desalination, wastewater treatment, emergency relief, and power generation. In forward osmosis (FO), an osmotic pressure gradient drives water across a semi-permeable membrane from a feed solution to a highly concentrated draw solution. Compared to pressure-driven membrane processes such as reverse osmosis (RO), FO operates at near zero hydraulic pressure, which can reduce membrane fouling and result in lower operating costs.
In some embodiments, thin film composite (TFC) polyamide RO membranes can include a polyamide active layer formed by interfacial polymerization on at least one support membrane such as, for example, polysulfone (PSF). The TFC membrane may also include an additional support membrane to enhance the mechanical stability of the PSF membrane underlying the active layer. When TFC membranes are used in FO applications, internal concentration polarization (ICP) is often generated within the thick, hydrophobic support layers of the membrane since mass transfer of water or salts across the membrane is substantially restricted by the support layer as well. ICP can significantly reduce the osmotic driving force across a membrane, which results in a substantial decrease in water flux. To reduce ICP in TFC membranes intended for FO applications, the backing layers of the TFC membranes can be modified to reduce overall membrane thickness. However, the hydrophobic nature of TFC support membranes such as PSF can still retard the mixing of water and salts, which can exacerbate ICP.
To be useful in FO applications, the wetting behavior of the porous support layers in the TFC membranes can also be enhanced by using hydrophilic polymers such as polybenzimidazole (PBI) and cellulose acetate (CA), but these hydrophilic support layers can swell when exposed to water, which can reduce mechanical stability of the support layers. Hydrophilic support materials can also interfere with interfacial polymerization processes used to form the polyamide active layer.